1. FIELD OF THE INVENTION
The present invention relates generally to a novel heat resistant steel, and more particularly to a novel gas turbine in which the heat resistant steel is used.
2. DESCRIPTION OF THE PRIOR ART
Cr-Mo-V steel is currently used in the discs for a gas turbine.
There has recently been a demand for improvement in the thermal efficiency of gas turbines from the viewpoint of the saving of energy. The most useful means of improving the thermal efficiency of a gas turbine is to increase the temperature and pressure of the gas used, and an improvement in the efficiency of about 3% in terms of relative ratio may be expected by raising the temperature of the gas used from 1,100.degree. C. to 1,300.degree. C. and increasing the pressure ratio from 10 to 15.
However, since the conventional Cr-Mo-V steel becomes insufficient in its strength in accompaniment with such high temperature and pressure ratio, a steel material having a higher strength is needed. Creep rupture strength has the biggest influence on the high-temperature properties and hence is a critical requirement with respect to the strength. Austenitic steel, Ni-based alloy, Co-based alloy and martensitic steel are generally known as structural material having level of creep rupture strength which is higher than that of Cr-Mo-V steel. However, Ni-based alloy and Co-based alloy are undesirable from the standpoint of hot workability, machinability, vibration damping property, etc. Austenitic steel is also undesirable since its high-temperature strength is not so high in the vicinity of temperatures between 400.degree. and 450.degree. C., as well as from the viewpoint of the entire gas turbine system. On the other hand, martensitic steel well matches other constitutent parts and also has a sufficient high-temperature strength. Typical martensitic steels have been disclosed in Japanese Patent Unexamined Publication No. 110661/83 and No. 138054/85, and Japanese Patent Examined Publication No. 2739/71. However, these materials are not necessarily able to achieve a high creep rupture strength at temperatures between 400.degree. and 450.degree. C., and further since the toughness of these materials after having been heated at high temperatures for long period of time is low, they cannot be used for turbine discs, so that an improvement in the efficiency of gas turbines cannot be achieved.
As is evident from the foregoing, if one uses a material merely having a high strength to cope with the high temperature and the high pressure involved with gas turbines, it is impossible to raise the temperature of the gas. In general, as the strength is increased, the toughness is decreased.